1. Field of the Invention
The present invention relates to a method for transferring and fixing a toner image, and more particularly to a method for transferring and fixing a toner image in which an intermediate transfer medium is utilized.
2. Description of the Prior Art
Commonly, in an image forming means having been provided with an intermediate transfer medium, a toner image held on a toner image carrier is formed through the development of an electrostatic latent image thereof by a developing toner, and is then transferred onto an intermediate transfer medium in the form of an endless belt or a roll, and the toner image thus transferred onto the intermediate transfer medium is further transferred again onto and then fixed on transfer material such as a transfer paper, and thus, an image is formed. According to such a means, it is possible to obtain a final image of high quality and additionally to introduce a so-called retention system, that is, a system in which a latent image once formed is utilized several times by repeating the development and the transferring thereof to form a great number of the identical final-images; this also results in the advantage that it is capable of providing high speed image formation. In addition to the above, it has also the other advantage of simplifying the development system, and of improving the image quality, and the like, because of the fact that an image transfer can be made onto a transfer material comprising a plain paper even when using a one-component conductive toner to serve as the developing toner. The various conditions required are very severe to properly accomplish the transfer of a toner image on the intermediate transfer medium onto the transfer material, and at the same time to accomplish the fixation thereof, and therefore it is quite difficult to accomplish the transfer and the fixation of an excellent toner image.
Heretofore, as has been described in Japanese Patent Examined Publication No. 41679/1971, for example, the means has been already known in which a toner image is transferred and fixed by the heat radiated from the transfer material that is heated, without heating the toner image on an intermediate transfer medium.
The aforesaid means is poor in thermal efficiency, so that it necessitates giving a considerably great amount of thermal energy to the transfer material, while it is advantageous in the aspects that there is no danger of overheating the intermediate transfer medium and it is capable of preventing the toner image carrier from causing a bad thermal influence. Besides, the amount of the thermal energy to be given to said transfer material varies in accordance with the kind of transfer material, that is, for example, a transfer material comprising a relatively heavier weight paper or a porous paper necessitates a great amount of thermal energy, and if the amount of thermal energy equivalent thereto is applied to a transfer material comprising a relatively higher weight paper or thinner one in thickness, it results in an overheated state and the transfer material is deformed, or a burn occurs. It is therefore necessary to control the thermal energy supply according to the kind of transfer materials so as to achieve an excellent transfer and fixing. Furthermore in the case that the toner image transfer and the fixation thereof are made at a high speed, it necessitates a further greater amount of thermal energy supply to the transfer material, with the result that the energy consumption is increased and there is a great danger of an outbreak of fire if a transfer material jamming was taken place.
On the other hand, as has been described in Japanese Patent Open to Public Inspection No. 78559/1974, there has been known the means in which a toner image on an intermediate transfer medium are heated up to the fusing temperature thereof without heating a transfer material, and thereby said toner image on the intermediate transfer medium is transferred onto and fixed on the transfer material.
In the aforesaid means, however, it becomes difficult to perform the fixing securely, because of the fact that the fluidity of the toner on the side coming into contact with the transfer material is worsened by collection of a large amount of heat radiated from the toner to the transfer material, and it is therefore necessary in practice to also properly heat up the intermediate transfer medium. Accordingly, heat is applied to the toner image carrier through the intermediate transfer medium, and in the case that the toner image carrier is an electrophotographic sensitive element, there occurs a decrease in image density caused by the lowering of initial potential, and a fog caused by the adhering of the material components of the intermediate transfer medium onto the surface of the toner image carrier, and it is impossible to obtain greater durability because the worsening of the intermediate transfer medium comes more quickly.
In order to avoid such troubles as described above, ways may be taken to cool the intermediate transfer medium in the moving passage area after the transfer and the fixing thereof, however, in the case of a forced cooling, a greater amount of energy is further consumed, and in the case of a spontaneous cooling, it is necessary to lengthen the moving passage to reach the toner image carrier, so that the equipment becomes larger in size.
In the processes of the transfer and the fixing of the toner, as already described, the toner on the side coming into contact with a transfer material is antecedently cooled and the fluidity thereof is worsened, and therefore, the cooled toner is not transferred completely to the transfer material and partially remain on the intermediate transfer medium as it is adhered thereto, so that the toner image carrier is contaminated and an offset phenomenon is also occurs.